Wheel bearing apparatus, that supports a wheel of a vehicle that freely rotationally supports a wheel hub to mount a wheel, via a double row rolling bearing, are utilized for driving wheels and driven wheels. For structural reasons, the inner ring rotation type is used for driving wheels and both the inner ring rotation type and the outer ring rotation type are used for driven wheels. There are four generation types of wheel bearing apparatus. The first generation type has a wheel bearing including a double row angular contact ball bearing fit between a knuckle, forming part of a suspension apparatus, and a wheel hub. The second generation type has a body mounting flange or a wheel mounting flange that is directly formed on the outer circumference of an outer member. The third generation type has one inner raceway surface directly formed on the outer circumference of a wheel hub. The fourth generation type has inner raceway surfaces formed on the outer circumferences of a wheel hub and an outer joint member, respectively.
In the bearing portion, seals are mounted to prevent leakage of lubricating grease sealed within the bearing and the entry of rain water or dust into the bearing from the outside. Recently, a wheel bearing intended to reduce the rotational torque to improve the fuel consumption has been developed. In the rotational torque, the ratio of the wheel bearings seal is high. That is, the sliding resistance is larger than the rolling resistance in the wheel bearing. Thus, it is desired to further reduce the rotational torque in the wheel bearing seal. In addition, it has been found, after verification of damages of recovered bearings, that the main causes for damaging the wheel bearings are due to defects in sealing rather than peeling of structural elements of the bearing. Accordingly it is believed that the operational life of the bearing can be extended by improving the sealability and durability of the seals of the bearings.
Various seals to improve sealability have been proposed. One representative example of a wheel bearing seal is shown in FIG. 11. The seal 50 includes a slinger 51 and an annular sealing plate 52 arranged opposite to each other. The slinger 51 is made of preserved steel sheet such as an austenitic-stainless steel sheet (JIS SUS 304 etc.). It is formed with a substantially L-shaped cross-section by a press process. The slinger 51 has a cylindrical portion 51a press fit onto the inner ring (not shown) to form a rotational side member. An upstanding portion 51b extends radially outwardly from the cylindrical portion 51a. 
The sealing plate 52 is formed with a substantially L-shaped cross-section. The sealing plate 52 is adapted to be mounted on an outer member forming a stationary member. The sealing plate 52 includes a metal core 53 and a sealing member 54 integrally adhered to the metal core 53 by vulcanized adhesion. The metal core 53 is made of preserved steel sheet such as an austenitic-stainless steel sheet. The metal core 53 includes a cylindrical portion 53a adapted to be fit into the end of the outer member and an upstanding portion 53b that extends radially inward from the cylindrical portion 53a. 
The sealing member 54 is formed from elastic material such as nitrile rubber etc. The sealing member 54 has first and second side lips 54a, 54b that contact with the upstanding portion 51b. The first and second side lips 54a, 54b are inclined radially outward from the upstanding portion 53b of the metal core 53. The side lips 54a, 54b are adapted to be in contact with the upstanding portion 51b of the slinger 51 with a predetermined interference (contacting loads). The provision of two side lips 54a, 54b provides sealability even if a conventional dust lip is eliminated.
In addition, the sealing member 54 is formed so that it extends both to the outer surface of the cylindrical portion 53a and the inner circumferential edge of the upstanding portion 53b of the metal core 53. This improves the sealability. A grease lip 54c projects toward the cylindrical portion 51a of the slinger 51. The tip of the grease lip 54c has a predetermined fastening force against the cylindrical portion 51a of the slinger 51.
The seal 50 is designed so that it has the following relationship between the fastening force P1 (i.e. a contacting load of the grease lip 54c) against the cylindrical portion 51a of the slinger 51 and the contacting load P2 (i.e. total of the contacting loads of both the side lips 54a, 54b) of the first and second side lips 54a, 54b. P1≧P2   (Relational Expression)
That is, the fastening force P1 of the grease lip 54c is set equal to or larger than the contacting load P2 of the first and second side lips 54a, 54b. The contacting load P2 is a force acting to separate the sealing plate 52 from the slinger 51. The fastening force P1 of the grease lip 54c is a force acting to resist against the contacting load P2 acting to separate the sealing plate 52 from the slinger 51. Accordingly, it is possible to prevent the separation of the sealing plate 52 from the slinger 51 by setting the fastening force P1 and the contacting load P2 to have the Relational Expression defined above.
Accordingly, it is possible to assemble the seal 50, formed as a unit of the sealing plate 52 and the slinger 51, into a wheel bearing without the need to separately assemble them into the wheel bearing. Thus, it is possible to improve the efficiency and workability during assembling of the seal 50 into the wheel bearing. See, Patent document 1: Japanese Laid-open Patent Publication No. 2007-211791.
However, in the prior art seal 50, if the fastening force P1 of the grease lip 54c is set too large, it is believed that not only the rotational torque of bearing is increased but also the durability of the bearing is detracted due to an increase of the bearing temperature caused by friction at the grease lip 54c. 